Stroke weight inconsistency - the same stroke weight is appearing different on the same artboard

I have an odd problem with inconsistency in stroke weight in illustrator. I have a stroke weight of .5 pixels in my illustrations - however when I placed another illustration on the page and gave it a .5px stroke weight it is appearing much thinner and I have to increase it to 1.5px for it to look the same. This is really frustrating as I need all stroke weights to be consistent.

Please let me know what the issue could be here.

Thanks, Rebecca